The present invention relates to digital communication and more particularly to synchronization of certain parameters between a receiver and a transmitter.
In an OFDM (orthogonal frequency division multiplexing) communication system, data is communicated in a series of time domain bursts. To form each time domain burst, an IFFT is applied to a group of frequency domain symbols and a cyclic prefix is added to the transform result prior to transmission. Transmission may involve conversion to an analog signal, conversion to an intermediate frequency (IF), then upconversion to a desired selectable carrier frequency prior to final amplification and propagation across a transmission medium. Upconversion is typically achieved by mixing the IF signal with a variable frequency oscillator signal. The carrier frequency is varied by varying the oscillator frequency.
On the receiver end, preamplification is followed by downconversion to IF from the carrier frequency, again by mixing with a variable frequency oscillator. The resulting IF signal is typically converted to a baseband digital symbol sequence. The cyclic prefix is removed and an FFT is applied to recover the original frequency domain symbols.
For successful communications, certain parameters must be synchronized between the transmitter and the receiver. For example, since OFDM communication proceeds on a burst by burst basis, the receiver and transmitter must agree on exactly when each burst begins. The consequence of missychronization will be lost data.
OFDM communications is particularly useful in environments where signals may take multiple paths from a transmitter to a receiver. OFDM communication systems are also often used in the presence of noise and/or interference.
What is needed is a burst synchronization system that may be used in OFDM communication systems and that performs robustly in the presence of multipath effects, interference, and noise.
The present invention provides a synchronization system useful in, e.g., OFDM communication systems. In one embodiment, a burst timing synchronization system may be impervious to the effects of dispersive channels and robust to noise and other interference. Special synchronization bursts may be used to both provide synchronization information and to carry data.
In accordance with a first aspect of the present invention, a system for transmitting an OFDM signal via a channel to facilitate receiver synchronization includes a transforming stage that transforms a first series of N frequency domain symbols into a first burst of N time domain symbols and that transforms a second series of M frequency domain data symbols into a second burst of M time domain symbols. A cyclic prefix appending stage appends to a beginning of the first time domain burst a first cyclic prefix duplicating a last segment of the first time domain burst to form a first synchronization burst, and appends to a beginning of the second time domain burst a second cyclic prefix duplicating a last segment of the second time domain burst to form a second synchronization burst. The first and second cyclic prefixes include a first portion having length v wherein v is greater than or equal to a duration of an impulse response of the channel and further include a second portion after the first portion to facilitate receiver synchronization. The first and second synchronization bursts are transmitted concatenated together.
In accordance with a second aspect of the present invention, a system for synchronizing to a received time domain signal includes a receiver system that receives one or more synchronization bursts of symbols. Each of the synchronization bursts includes at least two time domain synchronization sub-bursts. A first of the sub-bursts includes N time domain symbols preceded by a first cyclic prefix. A second of the sub-bursts includes M time domain symbols preceded by a second cyclic prefix. The first and second cyclic prefixes includes a first portion having length v wherein v is greater than or equal to a duration of an impulse response of the channel. The first and second cyclic prefixes further include a second portion after the first portion to facilitate receiver synchronization. The receiver system receives further time domain bursts of symbols. A synchronization block determines burst timing alignment responsive to optimization of a cost function determined responsive to the contents of the one or more synchronization bursts.
Other features and advantages of the invention will become readily apparent upon review of the following detailed description in association with the accompanying drawings.